The present invention relates to apparatuses and methods for protecting high-temperature batteries from extreme thermal excursions.
High temperature batteries have many desirable characteristics, including large energy and power density. However, failure of large batteries (which usually have a serial-parallel architecture) can be triggered by short circuit failures of individual cells. A short-circuited cell within the battery draws considerable amounts of current from other cells within the battery. These circulating currents individually need not be larger than those associated with providing power to an approved external load, but in this failure mode are all directed to heating the faulty cell.
The increased temperature associated with a short-circuited cell can cause nearby cells also to develop short-circuits. As more cells develop shorts, the circulating currents within the battery increases in a non-linear manner, driving temperatures above normal operating levels. This eventually produces a cascade effect of cell failure, and extreme thermal excursions, which can lead to catastrophic failure, including fire and/or release of toxic substances into the general environment.
The failure mode described above for high temperature batteries does not result from excess flow of electrical current, but rather from temperatures within the battery casing which are too high. If a cascade of failures occurs within a limited time, the battery will undergo a thermal meltdown.
It is possible to prevent such cascade failure by placing thermal disconnects between the cells. A thermal disconnect is a device which interrupts a circuit carrying a standard operational current when the environmental temperature exceeds a desired value. Note that this function differs from that of a conventional electrical fuse, which is a device which interrupts a circuit when the circuit carries a current in excess of a standard operational level.
This difference in function leads to key differences in structure. A conventional electrical fuse is intended to reliably permit currents below a rated value to pass, whereas currents a given amount larger than said rated value (typically 125% to 200% of the rated value) will cause the circuit to open. The specified rated value and the opening value are intended to be reliable for a range of operating temperatures.
A conventional electrical fuse comprises a conducting element which undergoes Joule heating from the current passing through the fuse, and hence through the conducting element. The conducting element is in thermal contact with (or may be identical to) a fusible link which has a known melting temperature. The conducting element is designed so that, when the current is equal to or less than the rated value, the temperature of the fusible link is less than its melting temperature. When the current is greater than the rated value, the temperature of the fusible link is greater than its melting temperature, resulting in opening of the circuit in which the fuse is placed.
A thermal disconnect requires additional structure to function as described above. In particular, connection to the circuit, mechanical mounting means, and the like provide thermal contact between the conducting element/fusible link combination and the external environment. For convenience, the structure leading to this thermal contact will be called a thermal link element.
A conventional electrical fuse cannot operate without a thermal link element. If a thermal link element is not present, then Joule heat produced in the conducting element by a current cannot dissipate. As a result, the temperature of the conducting element and the fusible link will steadily increase, eventually reaching the melting temperature of the fusible link. Thus, a conventional electrical fuse which does not have a thermal link element will open under any value of operating current, which is not the desired function.
If the thermal link to the surrounding environment provided by the thermal link element is too large, however, a conventional electrical fuse will again fail to function. This will become clearer if specific design parameters are used.
Consider an electrical fuse which is intended to have a rated value of 20 amperes, has a resistance of 0.1 ohm, is intended to open at a continuous overload of 25 amperes, and has a fusible link which melts at 250.degree. C. The environmental temperature of the apparatus is 25.degree. C. The Joule heating in the conducting element at the rated value is I.sup.2 R, or 40 watts, whereas at the opening value the Joule heating is 62.5 watts. The thermal link element has a thermal conductance K, expressed in units of .degree. C./watt, and the thermal power leaving the conducting element is K(T-t), where T is the temperature of the environment and t is the temperature of the conducting element. The equilibrium temperature of the conducting element is attained when the Joule heating is equal to the thermal power.
A current of the opening value must correspond to a conducting element temperature t greater than or equal to 250.degree. C., whereas a current of the rated value must give t&lt;250.degree. C. These requirements combined result in the design criterion 40K&lt;225.ltoreq.62.5K. K must therefore be between roughly 3.6 and 5.6.degree. C./watt. Note that at the smallest physically permissible value of K, the conducting element temperature at the rated current is about 170.degree. C., a considerable increase from the temperature of the surrounding environment. This increase under rated conditions is unavoidable in practical electrical fuses.
An electrical fuse which does not include in its structure a thermal link element of appropriate magnitude cannot function.
A thermal disconnect can be made which has a conducting element comprising a compound which melts above a given temperature T. and a thermal link element. However, the thermal link element for a thermal disconnect must be designed differently than that of an electrical fuse. This difference in design is sufficient that, in general, an electrical fuse cannot be used as a thermal disconnect, and vice versa.
Examine how a thermal disconnect must be designed. The intended function of a thermal disconnect is to open an electrical circuit when the temperature of the surrounding environment exceeds a design value T, essentially independent of the amount of current passing through the circuit. The simplest design consonant with this intended function is a fusible link through which the current of the electrical current flows and which melts at a temperature T, and a thermal link between the fusible link and the surrounding environment strong enough that the temperature of the fusible link is essentially independent of the amount of current flowing through the circuit.
This latter requirement is needed to prevent the operating conditions of the electrical circuit from causing the thermal disconnect to open below its operating temperature. It also clearly prevents such a thermal disconnect from operating as an electrical fuse.
To summarize, a thermal disconnect must have a structure producing a very strong thermal link between a fusible link and the surrounding environment, whereas an electrical fuse must have a structure producing a vastly weaker thermal link between a fusible link and the surrounding environment. This is a difference in scale, but one which produces a qualitatively different type of behavior.
There is a need for a new type of thermal disconnect suitable for application in high temperature batteries. This typically requires breaking an electrical circuit upon a component of a high temperature battery reaching a temperature indicative of failure of the component. In typical high temperature batteries this temperature is roughly between 400 and 500.degree. C.
A primary advantage is that a thermal disconnect according to the present invention is inexpensive in comparison to the total cost of a high temperature battery and in comparison to competing devices.